In optical recording CD systems, it is highly desirable to integrate the optical source, beam splitter, and a detector array into an integrated package generally referred to as a laser-detector-grating unit (LDGU). A number of LDGU-based systems are described in W. Ophey, "Compact Optical Light Paths," Jpn. J. Appl. Phys., Vol. 32, Part 1, No. 11B, pp. 5252-5257, November 1993. Other LDGU-based systems are described in, for example, U.S. Pat. Nos. 5,050,153 and 4,945,529.
The placement of the electronics necessary to support small form factor heads presents a number of technical challenges. Some of these challenges are to optimize the signal-to-noise ratio, minimize transmission line effects (to permit fast rise and fall times) and to provide signals that accurately represent focus and tracking front facet and RF sum. To meet some of these challenges, it is extremely important to have matched detectors as well as matched amplifiers, since focus and tracking error signals are usually generated by an algebraic function of two or more detector signals. Absolute matching, as well as good phase margins, are important for an accurate signal representation of derived focus or tracking error signals. In order to insure that the signals are accurately defined, it is necessary to include matching parameters such as transimpedance matching (typically 1%), voltage offset (typically 6 mv for critically matched amplifiers) and voltage offset drift with temperature typically (40 .mu.v/Deg C.) for critically matched preamplifiers.
In addition, high sampling rates place severe constraints on overload recovery time (typically &lt;25 nsec), settling time (&lt;10 nsec for 1% accuracy), bandwidth (typically 70 MHz min), and slew rate (typically 125 v/.mu.sec {volts per micro-sec}).
With the close proximity of preamplifiers and detectors come some rather tight coupling constraints (typically -45 to -55 db) for minimizing focus error crosstalk into tracking error signals and vice versa. In addition, noise parameters such as input referred current noise spectral density, typically 2-pa/sqrt (Hz) must be maintained.
Integrated circuit manufacturers today do not offer preamplifiers with the required matching voltage offset, noise, slew rate, bandwidth and recovery time parameters required to optimize performance in an optical recording CD system operating at 8-12.times. write speeds.
The present invention removes the foregoing limitations by forming a custom optical detection package utilizing an internal multi-chip module, a matched monolithic detector array, and preamplifiers monolithically fabricated in one custom ASIC.